title: Design and evaluation of an RF front-end for 9.4 T human MRI.
abstract: At the field strength of 9.4 T, the highest field currently available for human MRI, the wavelength of the MR signals is significantly shorter than the size of the examined structures. Even more than at 7 T, constructive and destructive interferences cause strong inhomogeneities of the B1 field produced by a volume coil, causing shading over large parts of the image. Specialized radio frequency hardware and B1 management methods are required to obtain high-quality images that take full advantage of the high field strength. Here, the design and characteristics of a radio frequency front-end especially developed for proton imaging at 9.4 T are presented. In addition to a 16-channel transceiver array coil, capable of volume transmit mode and independent signal reception, it consists of custom built low noise preamplifiers and TR switches. Destructive interference patterns were eliminated, in virtually the entire brain, using a simple in situ radio frequency phase shimming technique. After mapping the B1+ profile of each transmit channel, a numerical algorithm was used to calculate the appropriate transmit phase offsets needed to obtain a homogeneous excitation field over a user defined region. Between two and three phase settings are necessary to obtain homogeneous images over the entire brain.
authors.affiliation: High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany. shajan.gunamony@tuebingen.mpg.de
authors.forename: G
authors.initials: G
authors.lastname: Shajan
authors.forename: Jens
authors.initials: J
authors.lastname: Hoffmann
authors.forename: Juliane
authors.initials: J
authors.lastname: Budde
authors.forename: Gregor
authors.initials: G
authors.lastname: Adriany
authors.forename: Kamil
authors.initials: K
authors.lastname: Ugurbil
authors.forename: Rolf
authors.initials: R
authors.lastname: Pohmann
mesh_terms: D001921:Brain / Q000033:anatomy & histology*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008280:Magnetics / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014159:Transducers